This invention relates to devices for conveying bodily fluids, and more particularly to body implantable left ventricle or whole-heart assist pumps for bypassing the heart during open heart surgery, or to assist a disfunctional heart or left ventricle. More particularly, this invention relates to a drive means suited for such pumps.
Centrifugal blood pumps have long been recognized for their utility as a supplement to or replacement for the human heart, for example in assisting a damaged left ventricle, for temporary heart bypass if required in open heart surgery, and for total heart bypass when two such pumps are implanted. Such pumps operate continuously and at high speeds, for example in the range of about 4,000 to 7,000 rpm, and are relatively small to facilitate implantation.
One of the more challenging aspects associated with body implantable centrifugal pumps is to provide an appropriate means for driving the impeller. One approach is noted in U.S. Pat. No. 4,135,253 (Reich et al), directed principally to the use of a saline solution as a fluid lubricant to float a pump rotor within a rotor housing. A two-pole permanent magnet is secured to the rotor, and another two-pole permanent magnet is outside the body, rotatable by a power supply also outside the body. The outside drive magnet is positioned against the skin, axially spaced apart from but coaxial with the pump rotor, whereby rotation of the drive magnet rotates the driven magnet in the rotor.
One difficulty encountered in this approach is a substantial normal or axial force between the drive magnet and driven magnet, resulting in unwanted pressure on the skin immediately over the implanted pump. Further due to the axial force, the rotor requires a high efficiency spiral groove thrust bearing formed on its surface facing a flat carbon thrust pad in the pump casing.
Accordingly, a radial coupling of drive and driven magnets would be desirable as virtually eliminating the axial force. This, however, would require the pump to form an outward bulge in the skin, sufficient to accommodate positioning of a radial driver. Even assuming a bulge of sufficient size, it would be difficult to position a radial driver concentrically with respect to the pump, resulting in unbalanced radial forces acting on the rotor.
Yet another challenge presented in the centrifugal pump design is to provide a drive means conforming to the pump geometry. In addition to the usual constraints upon the size of any drive means, there is a need for a radial coupling structure with a diameter over three times its axial length. This geometry tends to discourage use of a conventionally wound stator as a means to drive a rotor assembly including a permanent magnet, since conventionally wound coils would favor a stator having an axial length greater than its diameter. Finally, in any body implantable device there is a need to minimize, to the extent possible, heat generated by the rotor drive means, and to provide for effective dissipation of generated heat.
Therefore, it is an object of the present invention to provide a means for driving a body implantable centrifugal pump through a radial magnetic coupling between a stationary drive means and a movable rotor.
Another object is to provide a stationary rotor drive means including two or more electrically conductive coils for two or more phases of a motor comprised of the rotor and drive, with the coils wound in a manner to minimize heat generated during operation.
Another object is to provide a drive structure for a rotor that is particularly well suited for dissipation of heat generated during its operation and for precisely determining the radial gap between the rotor and drive.
Yet another object of the invention is to provide a drive apparatus surrounding a rotor and radially spaced apart from the rotor an adequate amount to permit a hydrodynamic bearing type support of the rotor within a centrifugal pump.